Cyclic GMP has been implicated in the loss of growth control characteristic of human leukemic state. Recently, I have found that a factor from the bitter melon (Monordica charantia) preferentially inhibits guanylate cyclase activity and is specifically cytotoxic to leukemic vs. normal human lymphocytes. The factor lowers cellular cGMP levels and is a competitive inhibitor of guanylate cyclase. This suggests that elevated cGMP levels may be necessary for leukemic cell growth. Studies are being conducted to determine the mechanisms by which this nucleotide becomes and remains elevated in the leukemic cell.